In image forming apparatuses such as copiers, when a print key is turned on, a document set on a copy board is irradiated with light. A reflected light image from this document is projected on a photoelectric converter, e.g. a CCD (Charge Coupled Device), where it is converted into an electric signal.
The CCD has a light receiving surface on which a large number of light receiving elements (photoelectric converting elements) are arranged. The CCD scans each light receiving element to output an image signal (analog signal) with a voltage level corresponding to the density of the image projected on the light receiving surface.
In image forming apparatuses that can print color images, a red filter allowing red light to pass through, a green filter allowing green light to pass through, and a blue filter allowing blue light to pass through are arranged on the light receiving surface of the CCD. Accordingly, the CCD outputs an image signal corresponding to an image of red light, an image signal corresponding to an image of green light, and an image signal corresponding to an image of blue light.
The image signal corresponding to the image of red light is converted by an A/D (Analog-to-Digital) converter into a digital signal, which becomes image data R. The image signal corresponding to the image of green light is converted by the A/D converter into a digital signal, which becomes image data G. The image signal corresponding to the image of blue light is converted by the A/D converter into a digital signal, which becomes image data B.
On the other hand, image colors handled by the printer section include yellow, magenta, cyan, and black. Thus, the image data R, G, and B are converted into image data Y representing a yellow image, image data M representing a magenta image, and image data C representing a cyan image. Then, image data K representing a black image is generated on the basis of these image data Y, M, and C.
However, the image data Y, M, and C still contain elements of the above black image. To properly print the yellow image, the magenta image, and the cyan image, the elements of the black image contained in the image data Y, M, and C must be removed from the image data Y, M, and C.
Thus, it is necessary to have a processing section that removes the elements of the black image from the image data Y, a processing section that removes the elements of the black image from the image data M, and a processing section that removes the elements of the black image from the image data C.
However, the presence of the three processing sections disadvantageously complicates the configuration of the apparatus and increases its size and cost.